1. Field of the Invention
The present invention relates to a positive type photosensitive electrocoating composition having excellent running stability and highly reliable image formability, as well as to a process for pattern formation using the composition. More particularly, the present invention relates to a positive type photosensitive electrocoating composition suitable as an etching resist used in production of printed wiring board, as well as to a process for formation of electro-conductive pattern using the composition.
2. Description of the Prior Art
Printed wiring boards used in electronic apparatuses, etc. are produced mainly by a pattern printing process which comprises applying screen printing onto an insulating substrate having an electroconductive layer on the surface, or by a subtractive process which comprises forming a circuit pattern by photolithography using a photosensitive dry film.
Recently, as printed wiring boards have come to possess a higher density and higher accuracy, a finer circuit pattern and a smaller throughhole diameter have become necessary for such printed wiring boards. To satisfy such a need, there were proposed production processes using positive type photosensitive electrocoating resists.
As the positive type photosensitive electrocoating resist, there were proposed various water-soluble or water-dispersible compositions. Examples thereof are a composition composed mainly of a resin obtained by bonding o-quinone diazide sulfonic acid to a base resin (e.g. an acrylic resin having an ion-forming group) via a sulfonic acid ester linkage [U.S. Pat. No. 4,673,458 and corresponding Japanese Patent Application Kokai (Laid-Open) No. 206293/1986]; a composition composed mainly of a resin obtained by bonding hydroxylamine and o-quinone diazide sulfonic acid to a base resin (e.g. an acrylic resin having an ion-forming group) via a urethane linkage and a sulfonimide linkage [U.S. Pat. No. 4,975,351 and corresponding Japanese Patent Application Kokai (Laid-Open) No. 121375/1989]; a composition composed mainly of a mixture of (1) an acrylic resin obtained by copolymerization using, as one component, a polymerizable monomer having a relatively long side chain having an acid group (e.g. a carboxyl group) at the end and (2) a photosensitizer obtained by reacting an epoxy compound, an aromatic or heterocyclic carboxylic acid having a phenolic hydroxyl group, and a quinone diazide sulfonic acid halide [U.S. Pat. No. 5,055,374 and corresponding Japanese Patent Application Kokai (Laid-Open) No. 42446/1990]; a composition which is a mixture of (1 ) a compound or resin obtained by bonding hydroxylamine and o-quinone diazide sulfonic acid to a hydroxyl group-containing compound or resin via a urethane linkage and a sulfonimide linkage and (2) a base resin (e.g. an acrylic resin having an ion-forming group) [U.S. Pat. No. 5,134,054 and corresponding Japanese Patent Application Kokai (Laid-Open) No. 289660/1990]; a composition composed mainly of a photosensitive resin having a phenolic hydroxyl group and a carboxyl group or an amino group in the molecule, into which resin a quinone diazide sulfonic acid is introduced via a sulfonic acid ester linkage [U.S. Pat. No. 4,673,458, and corresponding Japanese Patent Application Kokai (Laid-Open) No. 206293/1986]; a composition comprising a mixture of a carboxyl group-containing polymer and a phenolic hydroxyl group-containing polymer, into which an o-quinone diazide sulfonic acid is introduced via a sulfonic acid ester linkage [Japanese Patent Application Kokai (Laid-Open) No. 100073/1991]; and a composition composed mainly of a mixture of a carboxyl group-containing acrylic resin and a sulfonic acid ester between a phenolic hydroxyl group-containing aromatic carboxylic acid ester and quinone diazide sulfonic acid [Japanese Patent Application Kokai (Laid-Open) No. 100074/1991].
The above conventional compositions, however, have various problems as mentioned below, and solutions thereto are needed.
For example, in the resin having a photosensitive group introduced via a sulfonic acid ester group, it is necessary to react the hydroxyl group (e. g. phenolic hydroxyl group) of a resin, with a sulfonic acid halide by dehydrohalogenation to form a sulfonic acid ester and thereby introduce an o-quinone diazide group. This reaction, however, is a polymer reaction and has various problems such as difficulty of quantitative introduction of said photosensitive group, difficulty of removal of halogen acid salt (by-product) and consequent difficulty of purification, long time required for production, low product yield and the like. Thus, the reaction is not suitable for industrial application.
In the process for introducing a photosensitive group, proposed in order to avoid a polymer reaction giving a by-product, which comprises reacting an o-naphthoquinone diazide sulfonic acid halide with a hydroxylamine derivative to bond them via a sulfonamide linkage, reacting the product with a diisocyanate (e.g. toluene diisocyanate) to form an intermediate having an isocyanate residue and a photosensitive group, and reacting the intermediate with a hydroxyl group-containing resin, there are advantages such as high yield in polymer reaction, no need of purification step, and the like; however, the steps up to formation of intermediate are complicated and take a long time, which poses the same problems as mentioned above.
In the resin obtained by introducing a photosensitive group into the hydroxyl group of a hydroxyl group-containing resin, the photosensitive group is randomly arranged on the resin; there fore, when the resin is made into an aqueous dispersion, the portion of the resin containing the photosensitive group of high hydrophobicity tends to be present on the surface of each resin particle of the aqueous dispersion. As a result, the aqueous dispersion is unstable and tends to cause deposition; and the azide group contacts easily with a basic compound (e.g. an amine) present in a large amount in the continuous phase of the aqueous dispersion, which easily invites the decomposition of the azide group and gives reduction in photosensitivity. This is a big drawback for an electrocoating resist which is required to have aqueous dispersion stability over a long term.
In the resin having a photosensitive group introduced via a sulfonic acid ester group, the sulfonic acid ester group is hydrolyzed easily, inviting further reduction in storage stability of electrocoating resist as well as in running stability of electrocoating bath.
In the process which comprises introducing a photosensitive group via a sulfonic acid imide linkage and a urethane linkage, the problem associated with hydrolysis is improved significantly, but there are problems such as (1) the dissolution of the irradiated portion of film in developer is slow, (2) development must be conducted at high temperatures and (3) the conditions allowing for development have a slightly narrow range.
As one measure for solving these problems, there were proposed electrocoating resists which are each an aqueous dispersion of a composition comprising a water-soluble or water-dispersible resin and an o-quinone diazide type photosensitizer; and a composition comprising an alkali-soluble resin (e.g. a phenolic resin) and an o-naphthoquinone diazide sulfonic acid ester of a polyhydroxybenzophenone is already in wide use as a resist for LSI production. In using this composition as an electrocoating resist for production of printed wiring board, the phenolic resin is not desirable in view of the dispersibility in water and the properties of the film obtained; therefore, a composition was proposed which uses an acrylic resin or the like as a water-soluble or water-dispersible resin. In these compositions, however, when they are made into aqueous dispersions, part of the quinone diazide type photosensitizer is present on the surfaces of the resin particles of the aqueous dispersion and the contact of the azide group with a basic compound gives rise to the deposition of the azide group, inviting reduction in photosensitivity.
Further, when a photosensitizer of sulfonic acid ester type is used, the sulfonic acid ester undergoes hydrolysis and the stability of the aqueous dispersion prepared is deteriorated, making it difficult to obtain an electrocoating resist bath having excellent running stability over a long term.
In order to improve the stability of an aqueous dispersion, it was proposed as mentioned above to use a comb-shaped acrylic resin obtained by copolymerization of a particular monomer of relatively long chain polyester or polyether having an acid group at one end and a polymerizable unsaturated group at the other end (in this resin, the ion-forming group is distant from the resin skeleton which is hydrophobic) and thereby allow a highly hydrophobic quinone diazide type photosensitizer to be easily present inside the resin particles dispersed in water. By this approach, there can be obtained an aqueous dispersion having considerably improved storage stability, but its running stability over a long term is not sufficient. Further in order to obtain said acrylic resin, a special monomer as mentioned above must be used and the process for production thereof has more and complicated steps and requires a long time. Furthermore, said monomer has a relatively large molecular weight and low copolymerizability with ordinary acrylic ester type monomers, etc., and tends to remain unreacted in produced resin or form a homopolymer.
The unreacted monomer and homopolymer tend to act as water-soluble components when present in an aqueous dispersion of composition and enables the formation of an aqueous dispersion of relatively good storage stability. This aqueous dispersion, however, has problems as an electrocoating resist because, when it is used in electrocoating and there is conducted circulation of electrocoating solution by pump (this circulation is requisite in electrocoating), the dispersed resin particles tend to cause fusion with each other and generate an agglomerate in the solution, which easily causes (1) deposition of said agglomerate on formed resist film and (2) plugging of filter used in circulation system. Further, since the composition of the above proposal uses a photosensitizer obtained by reacting an epoxy compound, an aromatic or heterocyclic carboxylic acid having a phenolic hydroxyl group and an o-quinone diazide sulfonic acid halide, many steps are needed in production of said photosensitizer, which is not favorable in industrial application.
Also in an electrocoating resist which is an aqueous dispersion of a mixture comprising a carboxyl group-containing polymer and a photosensitizer obtained by a quinone diazide type photosensitive group to a phenolic hydroxyl group-containing polymer (e.g. a polyvinylphenol) via a sulfonic acid ester linkage, the running stability is insufficient similarly to the above-mentioned case. Moreover, the photosensitizer used therein is obtained by a polymer reaction and therefore has the same production problems as mentioned above.